In general, an HVDC transmission system (high voltage direct current transmission system) is a method where electrical power is transmitted by converting alternating current generated from a power plant into direct current and then supplied by reconverting the direct current into alternating current at the service entrance point. The high voltage direct current transmission system suffers lower electrical losses than an AC transmission system, may have improved stability through line separation, and is suitable for long-distance transmission due to less inductive disturbance.
In a high voltage direct current transmission system, a plurality of sub-modules is mounted to a structure called a converter module, which is ten meters high and has a plurality of stories. The sub-module is a heavy object weighing more than 200 kg, so an aerial ladder truck is required for maintenance work on the sub-module. An aerial ladder truck is generally used because it is impossible to mount an overhead travelling crane to a substation, such as a converter module.
In the case where an aerial ladder truck is used, moving the heavy sub-module to the aerial ladder truck from the high rise structure is dangerous and difficult. The aerial ladder truck is generally powered by hydraulic pressure, so even when the load capacity is higher than the weight of the sub-module, the sudden shift of the weight caused by the heavy material makes center of gravity of the aerial ladder truck suddenly change, thereby causing a safety problem.
Further, considering the weight of the sub-module, it is very dangerous and difficult for a worker to move the sub-module to the aerial ladder truck in the structure. In particular, it is hard and dangerous for the worker on the aerial ladder truck to withdraw the heavy sub-module from the structure and load the module to the aerial ladder truck.